1. Field of the Invention
The present invention relates to a NOx sensor, and more particularly to a NOx sensor for detecting the total concentration of nitrogen oxides in combustion exhaust gases.
2. Description of the Related Art
In recent years much attention has been paid to solid type gas sensors which can be directly inserted into automotive and other engine exhaust gases for continuous measurement. Many reports have been made on the research and development thereof. The present inventors have already proposed mixed potential type NOx sensors which are capable of real-time measurement of the total NOx concentration in exhaust gases. For example, Japanese Patent Laid-Open Publication No. Hei 11-23526 describes a configuration in which a NOx sensing electrode of mixed potential type is installed in a gas detection chamber composed of an internal space formed by zirconia solid electrolytes, and a NOx conversion electrode (oxygen pumping cell) is opposed thereto within the same detection chamber. That is, NOx (NO and NO2) in the exhaust gas is electrochemically converted into simple NO2 gas so that the NOx sensing electrode detects the NO2 concentration as the total NOx concentration.
In the Japanese Patent Laid-Open Publication No. Hei 11-23526, a NOx conversion pumping cell for converting NOx into N)2 in the detection chamber is provided along with the oxygen pumping cell for adjusting the oxygen concentration within the detection chamber. Besides the aforementioned NOx sensing electrode and reference electrode, an oxygen sensing electrode for measuring the oxygen concentration in the chamber is also installed in the detection chamber. In this sensor configuration, the potential of the oxygen sensing electrode resulting from the oxygen concentration in the same atmosphere is used as the reference potential of the NOx sensing electrode so that the output of the NOx sensor is less subject to variations in the oxygen concentration of the gas detection chamber.
Nevertheless, in for example the cases of automotive exhaust gas, there also exists such interference gases as hydrocarbon gas (HC) and CO which tend to affect the detection accuracy of the NOx concentration to be measured. Thus, in order for the conventional sensor device to maintain a higher oxygen concentration in its detection chamber, the oxygen pumping cell has been installed at the prior stage of the NOx detection chamber so that oxygen is supplied into the detection chamber while HC and the like are oxidized and removed on the surface of an oxygen pumping electrode (anode electrode).
In other words, the intention has been that the catalysis of the oxygen pumping electrode installed at the prior stage of the detection chamber and the catalysis of the zirconia solid electrolytes forming the detection chamber oxidize such interference gases as HC and CO which approach the detection chamber. The interference gases are thereby converted into H2O and CO2 which have no effect on the NOx detection.
In the conventional method, however, if concentrations of the reducing gases or the gases to be oxidized and removed in the gas to be detected are too high, the catalysis-based oxidation and removal described above becomes more difficult. More specifically, HC, CO, and others that are left neither oxidized nor removed come into contact with the NOx sensing electrode at the subsequent stage of the detection chamber, thereby giving an output reverse to the NO2 detection output. Accordingly, there has been a high possibility that the detection of the total NOx concentration might become inaccurate if the reducing gases are thus insufficiently removed.
As described above, in the conventional sensor structure, it has been impossible to oxidize and remove HC and other reducing gases in the gas to be detected sufficiently by simply maintaining a high oxygen concentration in the detection chamber, and utilizing the catalytic activity of the oxygen pumping electrode and the zirconia solid electrolyte and the like. Accordingly, there has been a problem that when the gas to be detected contains high amounts of reducing gases such as HC, the total NOx concentration cannot be measured accurately. Therefore, means has been desired which can measure the total NOx concentration accurately even if high concentrations of reducing gases such as HC and CO coexist.
In order to achieve the object aforementioned, there is provided a NOx sensor comprising: a gas detection chamber composed of an internal space surrounded by zirconia solid electrolyte substrates having oxygen ion conductivity; a NOx sensing cell including a NOx sensing electrode fixed onto the zirconia solid electrolyte substrate in the gas detection chamber, the NOx sensing electrode being active to NOx and oxygen, and a reference electrode fixed onto the zirconia solid electrolyte substrate, the reference electrode being active to at least oxygen; a NOx conversion pumping cell including a NOx conversion electrode fixed onto the zirconia solid electrolyte substrate in the gas detection chamber, the NOx conversion electrode being active to NOx and oxygen, and a counter electrode to be paired with said NOx conversion electrode, the counter electrode being fixed onto the zirconia solid electrolyte substrate, being active to oxygen; voltage applying means for applying a predetermined voltage to the NOx conversion pumping cell; a first gas treatment chamber communicating with the gas detection chamber and having a gas inlet leading to an atmosphere of a gas to be detected, an inorganic porous member being loaded into the first gas treatment chamber; and means for measuring a potential difference between the NOx sensing electrode and the reference electrode while converting NOx in the gas to be detected into single component after a reducing gas in the gas to be detected is oxidized in the first gas treatment chamber, and thereby detecting a total NOx concentration in the gas to be detected. In the sensor of the present invention, the first gas treatment chamber loaded with the inorganic porous member is arranged at the prior stage of the gas detection chamber. Then, the porous member of the first gas treatment chamber is filled with a high concentration of oxygen to be supplied to the internal space of the device by an oxygen supplying pumping cell or the NOx conversion pumping cell. Reducing gases are therefore oxidized and removed by the catalysis of the porous member.
In a preferred embodiment of the NOx sensor according to the present invention further comprising:
an oxidation catalyst pumping cell including an oxidation catalyst electrode composed of the inorganic porous member loaded into the first gas treatment chamber, the oxidation catalyst electrode serving as an anode electrode, and a cathode electrode to be paired with the oxidation catalyst electrode, the cathode electrode being arranged on a zirconia electrolyte substrate outside the gas detection chamber and being active to oxygen; and voltage applying means for applying a predetermined voltage to the oxidation catalyst pumping cell. In this configuration, the gas to be detected, containing reducing gases, flows into the gas detection chamber while making contact with the oxidation catalyst electrode. Thus, the reducing gases in the gas to be detected are electrochemically oxidized on the oxidation catalyst electrode. That is, the provision of the porous oxidation catalyst electrode active to the reducing gases and the supply of active oxygen to the oxidation catalyst electrode force the reaction to occur on the oxidation catalyst electrode so as to prevent HC from flowing into the gas detection chamber.
In another embodiment of the NOx sensor according to the second aspect of the present invention, at least one narrow path, or a gas diffusion path, is arranged between oxidation catalyst electrodes placed in the first gas treatment chamber. Here, the catalyst electrodes need not be porous, but it is desirable that they are porous in terms of performance. In this configuration, the gas to be detected, containing reducing gases, flows into the gas channel in the oxidation catalyst electrode while supplying the oxidation catalyst electrode with active oxygen using the oxidation catalyst pumping cell so as to effectively oxidize and remove the reducing gases.